1. Field of the Invention
The present invention relates to an optical recording/reproducing apparatus which realizes an optimal tracking servo-control according to the type of a recording medium such as an optical disc, and a method of detecting a tracking error signal.
2. Description of the Related Art
In general, a reproducing apparatus performs a tracking servo-control in a three-beam method, and a recording apparatus performs a tracking servo-control in a push-pull method, in particular, in a differential push-pull (DPP) method which is an improved push-pull method. Both the three-beam method and the DPP method utilize beams diffracted to the 0th and ±1st orders by a grating.
In a reproducing apparatus using the three-beam method, a grating in which the ratio in effectiveness of diffraction between the 0th and ±1st order beams is about 4:1 through 5:1. That is, the ratio of (0th order:±1st order)=(4:1) through (5:1) is adopted according to the intensity of a beam during a reproduction. The difference in phase between the ±1st order beam and −1st order beam irradiated on an optical disc is set to be 180°.
FIG. 1 shows a six-section photodetector 10 of a conventional reproducing apparatus. The six-section photodetector 10 includes a main photodetector 11 having a four-section structure and a pair of sub-photodetector 13 and 15 arranged at corresponding sides of the main photodetector 11. The tracking servo-control is realized in the three-beam method by detecting three beams diffracted by a grating. Here, a tracking error signal detected in the three-beam method is a differential signal between detection signals of the sub-photodetectors 13 and 15.
In a recording apparatus using the DPP method, a grating in which the ratio in effectiveness of diffraction between the 0th and ±1st order beams of about 10:1 through 15:1 is adopted to increase the efficiency of the 0th order beam used for a recording. The difference in phase between the +1st order beam and −1st order beam irradiated on an optical disc is set to be 360°.
FIG. 2 shows an eight-section photodetector 20 of a conventional recording apparatus. The eight-section photodetector 20 includes a main photodetector 21 having a four-section structure and a pair of two-section sub-photodetector 23 and 25 arranged at corresponding sides of the main photodetector 21. The tracking servo-control is realized in the DPP method by detecting three beams diffracted by a grating. Here, a tracking error signal detected in the DPP method is the difference between a sum signal of detection signals of sections I1 and J1 of the sub-photodetectors 23 and 25 and a sum signal of detection signals of sections I2 and J2 of the sub-photodetectors 23 and 25.
The tracking error signals detected in the three-beam method and the DPP method have a different magnitude according to the depth of a pit in an optical disc. FIG. 3 shows that the magnitude of a tracking error signal (TES3-BEAM) in the three-beam method becomes maximum as the depth of a pit of an optical disc is λ/4. In contrast, the magnitude of a tracking error signal (TESDPP) in the DPP method becomes maximum as the depth of a pit of an optical disc is λ/8, while the magnitude becomes minimum as the depth of a pit of an optical disc is λ/4.
Therefore, the depth of a pit of an optical disc is standardized to an intermediate value, for example λ/5, so as to realize the tracking servo-control for any one of the above servo-control methods adopted.
However, many of the optical discs currently being sold are manufactured so as to have a pit depth of closer to λ/4 which is deeper than the standardized size. As data is reproduced from an optical disc having deeper pits than the standardized size, the magnitude of a tracking error signal detected by a reproducing apparatus using the three-beam method is large whereas the magnitude of a tracking error signal detected by a reproducing apparatus using the DPP method is close to 0. Accordingly, the tracking servo-control itself becomes impossible.